2. Description of the Related Art
A CDMA (Code Division Multiple Access) system is one of the multiple access system technologies used in radio communication systems using mobile telephones and portable telephones, etc. when a plurality of stations carry out communications simultaneously on a same frequency band.
The CDMA system implements multiple accesses by spread spectrum communications in which information signals are transmitted with their spectrum spread over a sufficiently wide band relative to the original information bandwidth. In the CDMA system, there are several ways to implement the spread spectrum system above. Among them, a direct sequence system is a system in which a spreading code which has a faster transmission rate than the transmission rate of an information signal is directly carried on the information signal at the time of spreading. In this case, signals from a plurality of mobile stations are multiplexed on a same frequency area and same time zone.
The CDMA system using direct sequence has a so-called xe2x80x9cnear-farxe2x80x9d problem. This xe2x80x9cnear-farxe2x80x9d problem occurs when a desired transmitting station is located far and another undesired transmitting station (interfering station) is near, which causes the reception power of a received signal from the interfering station to be greater than that from the desired transmitting station, preventing the stations using only processing gain (spreading gain) from suppressing correlation between spreading codes, which disables communications. For a cellular system using the direct sequence CDMA system, it is therefore indispensable to control transmission power according to the state of each channel condition on the uplink from a mobile station to a base station.
Furthermore, as a countermeasure for fading which is the cause of degradation of the BER(Bit Error Rate) in terrestrial mobile communications, a method for compensating variations of instantaneous values of reception power by controlling transmission power is proposed.
The operation of closed-loop transmission power control processing is explained using a conventional slot configuration. FIG. 1 shows a slot configuration on a time scale when carrying out conventional transmission power control.
Signals consisting of pilot data 1, transmission power control data (TPC) 2 and transmission data 3 which are time-multiplexed in slot units are transmitted from a base station. Pilot data 1 is a signal with a fixed information pattern and is used by a mobile station to estimate channel conditions for demodulation and measure SIR (signal to interference ratio) and transmission power control data 2 is used as a transmission power control command.
Signals on the uplink from a mobile station to a base station as well as signals on the downlink from a base station to a mobile station are transmitted as slot cycle signals and a xc2xd slot timing offset (TShift) is added to the downlink to minimize a transmission power control delay.
First, transmission power control performed on the downlink is explained. A signal sent from a base station is received by a mobile station with a propagation delay of TDelay (corresponding to the distance from the base station to the mobile station). The mobile station measures reception SIR according to pilot data 4 at the start of a slot. Then, the mobile station compares this SIR measurement result with a given reference SIR and if the reception SIR is lower, generates a transmission power control bit which instructs the base station to increase transmission power, and if the reception SIR is higher, generates a transmission power control bit as a command to instruct the base station to lower transmission power. This transmission power control bit is embedded as transmission power control data 5 on the uplink and transmitted.
A signal sent from the mobile station is received by the base station with a delay of TDelay. The base station detects transmission power control data 6 and determines the transmission power value on the downlink from the result and reflects it in the transmission power at the start of the next downlink slot.
Then, the operation of transmission power control performed on the uplink is explained.
A signal sent from the mobile station is received by the base station with a delay of TDelay. The base station measures SIR according to pilot data 7 at the start of a slot and compares the reception SIR and a reference SIR as in the case of the mobile station, generates a transmission power control bit which is a command to instruct whether to increase/decrease transmission power and embeds it in transmission power control data 8 on the downlink and transmits it.
A signal sent from the base station is received by the mobile station with a delay of TDelay. The mobile station detects transmission power control data 9 and determines the transmission power value on the uplink from the result and reflects it in the transmission power at the start of the next uplink slot.
Since the uplink slot has a timing offset of xc2xd slot with respect to the downlink slot, transmission power control is carried out with one time slot control delay (the result of control 1 slot before is reflected) for both the downlink and uplink.
Then, a case where the transmission rate is lowered is explained with reference to FIG. 2. When the transmission rate is lowered, the absolute time of 1 bit (or symbol) becomes longer, which increases the ratios of pilot data length and transmission power control bit length to the slot length.
In this case, as the case above, signals 11 to 13 sent from the base station are received by the mobile station with a propagation delay of TDelay (corresponding to the distance from the base station to the mobile station) and the mobile station measures the reception SIR from pilot data 14 at the start of a slot. The mobile station compares this SIR measurement result with a reference SIR and embeds the result as transmission power control data 15 on the uplink and transmits it.
A signal sent from the mobile station is received by the base station with a delay of TDelay. The base station detects transmission power control data 16 and determines the transmission power value of the downlink from the result and reflects it in the transmission power at the start of the next downlink slot.
A signal sent from the mobile station on the uplink is received by the base station with a delay of TDelay. The base station measures SIR according to pilot data 17 at the start of a slot and compares the reception SIR with a reference SIR as in the case of the mobile station, generates a transmission power control bit which is a command to instruct whether to increase/decrease transmission power and embeds it in transmission power control data 18 on the downlink and transmits it.
A signal sent from the base station is received by the mobile station with a delay of TDelay. The mobile station detects transmission power control data 19 and determines the transmission power value.on the uplink from the result and reflects it in the transmission power at the start of the next downlink slot.
With low rate transmissions in the conventional apparatus, however, the ratios of the pilot data length and transmission power control bit length to the slot length may increase, causing a transmission power control delay due to a closed loop to increase. When a transmission power control delay increases, transmission power control is not reflected in the next slot, disabling appropriate transmission power control according to changes in a communication environment.
It also has a problem that minimizing a control delay would lead to a reduction of the SIR measurement time used for transmission power control, making it impossible to achieve the sufficient measurement accuracy.
It is an objective of the present invention to provide a base station apparatus and transmission power control method capable of minimizing control delays in closed-loop transmission power control and suppressing deterioration of the measuring accuracy due to reduced SIR measurement time.
The inventor came to come up with the present invention by discovering by focusing on a data slot configuration that adequately changing the arrangement of a slot configuration could prevent transmission power control from failing to catch the next slot when pilot data and transmission power control data are relatively long.
In this case, the arrangement of the slot configuration is performed by taking into account a propagation delay from a base station to a mobile station, the pilot data length for measuring the quality of the received signal, and time factors that could possibly cause control delays including the processing time after the mobile station has received the last part of the pilot data until it measures the quality of the received signal and embeds transmission power control data, a propagation delay from the mobile station to the base station, and the processing time after the base station has received the transmission power control data until it detects the transmission power control data and changes the power, etc.